1. Field of the Invention
The present invention relates to a charging roll for use in an image forming apparatus such as an electrophotographic copying machine, laser beam printer, or the like.
2. Discussion of the Related Art
There is known a charging roll which is installed in an image forming apparatus such as an electrophotographic copying machine or printer, such that the charging roll is held in rolling contact with a photosensitive drum for charging the circumferential surface of the photosensitive drum. More specifically described, such a charging roll is used in a so-called "roll charging" method which is one of the known methods for charging a photosensitive drum on which an electrostatic latent image is formed. In the roll charging method, the charging roll to which a charging voltage is applied is held in pressing contact with the outer circumferential surface of the photosensitive drum. The charging roll and the photosensitive drum are rotated together so that the outer circumferential surface of the photosensitive drum is evenly charged by the charging roll before the surface is locally imagewise exposed to optical image signals.
In general, the charging roll includes an electrically conductive center shaft (metal core) and an electrically conductive rubber layer which has a low hardness and is formed of a rubber layer. The rubber layer consists of either a solid elastic body whose hardness is reduced by adding a large amount of softener, or a foamed body. The electrically conductive rubber layer is formed on the outer circumferential surface of the center shaft with a suitable thickness. On the outer circumferential surface of the conductive rubber layer, there are laminated a resistance adjusting layer and a protective layer in this order. An electrode layer is interposed, as needed, between the conductive rubber layer and the resistance adjusting layer.
In the charging roll constructed as described above, there has been employed the resistance adjusting layer which is formed of a rubber composition prepared by mixing a suitable rubber material with an ion- conductive material such as quaternary ammonium salt, so as to give the rubber composition a desired volume resistivity. However, the resistance adjusting layer containing the ionconductive material tends to suffer from a variation of its characteristics due to a change of the operating environment of the charging roll. To solve this problem, it is considered to form the resistance adjusting layer by using a rubber composition which includes electron-conductive particles such as carbon black particles as a conductive material.
However, the use of the charging roll whose resistance adjusting layer contains the electron-conductive particles deteriorates a quality of a reproduced or printed image, when the surface of the photosensitive drum has flaws such as pinholes, or any scratched, warred or otherwise damaged or defective portions. Namely, the reproduced image undesirably includes printing defects (e.g., pinhole defect) corresponding to the defective portions even when the surface defects of the photosensitive drum are not so considerably serious. For instance, an image area corresponding to a pinhole and its vicinity tends to be blurred.